1. Field of the Invention
This invention relates in general to means and methods for producing chemical vapor deposition (CVD) reactions on seed particles and, more particularly, to improved means and methods for converting silicon and similar materials from gaseous to solid form.
2. Background of the Invention
High purity silicon, germanium, and like materials are much used for the production of semiconductor devices. High purity silicon is typically obtained, for example, by thermal decomposition of silicon bearing gases which have been refined to remove, insofar as possible, all impurities. The thermal decomposition is most easily accomplished on a heated surface which becomes coated with the elemental silicon released from the silicon bearing gases. This process is called chemical vapor deposition (CVD). The avoid introducing extraneous contamination, it is desirable that the starting or "seed" surface on which CVD occurs also be made of high purity silicon.
Fluidized bed reactors are a useful means of bringing large numbers of solid particles in intimate contact with a reactant gas. They have long been used for the production of gaseous silicon bearing compounds by passing etchants such as HCl vapor through a fine bed of silicon particles, to produce gaseous silanes and chlorosilanes which can then be readily purified. More recently, attempts have been made to use fluidized beds as a means for extracting silicon from purified silane and chlorosilane compounds.
If a silicon bearing gaseous compound is passed through a fluidized bed of silicon seed particles at a temperature of about 1,000.degree. C., the silicon bearing compound will decompose, depositing elemental silicon on the seed particles, which thereby increase in size. However, when fluidized beds are used for the production of elemental silicon, as opposed to silicon bearing gases, great practical difficulties are encountered. In particular, as the silicon particles in the bed grow in size they tend to agglomerate into clumps, effectively glued together by the silicon being deposited on their surfaces from the CVD reaction. This agglomeration leads to a reduction in the throughput and efficiency of the reaction since the agglomerated particles have a smaller total surface area than the separate individual particles, and the reaction rate and conversion efficiency depend upon the total available surface area.
A further area of difficulty which has been encountered with the use of fluidized beds of harvesting of silicon is that the fine holes which are present in the particle support plate of the typical fluidized bed reactor, become clogged with deposited silicon or with silicon particles. This reduces the throughput capacity of the bed and requires frequent shut-downs to clean out the holes or replace the plates. A particle support plate is a gas permeable plate placed at the bottom of a fluidized bed to support the particles in the bed while allowing gas to pass through and create the boiling-like turbulance, that is characteristic of a fluidized bed.
An additional problem associated with fluidized silicon harvesting beds of the prior art is that there is no effective means, short of stopping the reaction and emptying the bed, for separating those particles on which a substantial amount of silicon has been deposited from those particles which have grown in size by agglomeration but on which relatively little silicon has been deposited. Both have about an equal tendency to settle to the bottom of the bubbling mass of particles in the bed. Thus, agglomeration greatly interferes with one's ability to separate particles on which significant deposition has occurred. As a consequence, the silicon removed from the prior art reactors contains a larger proportion of seed material than is desired. This increases the cost of operation, both for the additional seed material required per unit weight of output and for the additional energy wasted in heating the additional seed material.
For these and other reasons, it has not proved practical to operate fluidized bed silicon harvesting systems efficiently and in a continuous fashion without encountering substantial operating and maintenance problems.
Accordingly, it is an object of the present invention to provide an improved CVD reactor system in which finely divided seed particles are brought in contact with a reactant gas comprising the material to be deposited.
It is an additional object of the present invention to provide an improved CVD reactor system in which particle agglomeration is minimized or avoided, and which readily permits differential separation of reacted and non-reacted particles so that those particles on which significant CVD has occurred may be readily harvested while others are left to react further.
It is a further object of the present invention to provide a reactor system of improved thermal configuration for greater energy efficiency.
It is an additional object of the present invention to avoid the problems associated with the clogging of particle support plates by avoiding the use of such plates.
It is a further object of the present invention to provide an improved method for the harvesting of silicon from silicon bearing gases within a CVD reaction system.
It is an additional object of the present invention to provide an improved method for harvesting silicon, particularly semiconductor grade silicon, from silicon bearing gases which can be operated in a substantially continuous manner.
It is a further object of the present invention to provide an improved reactor system in which additional seed particles can be added, in which particles already in the system can be circulated through multiple CVD cycles, and in which reacted particles can be harvested without substantially interrupting operation of the reactor systems.